1. Field of the Invention
The present invention relates to a method of injection molding articles of thermoplastic materials. The method includes initially forcing molten plastics material into an injection mold in an amount sufficient for forming the article. Subsequently, a flowable medium, particularly gas, is forced into the injection mold and/or the thermoplastic material with a pressure which distributes the plastics material uniformly over the surface of the mold cavity so as to form a hollow body. Further, the hollow body is cooled in the injection mold while the pressure of the medium is maintained. Finally, the pressure of the medium is released from the hollow body and the molded article is released and removed from the injection mold.
The present invention also relates to an apparatus for carrying out the method described above. The apparatus includes a pressure reservoir for the medium which can be connected to the injection mold through lines and control and/or regulating elements provided in the lines.
2. Description of the Related Art
For injection molding hollow articles of thermoplastic material, German Offenlegungsschrift No. 25 01 314 describes a method in which, immediately after injecting molten plastics material into an injection mold in an amount sufficient for forming a molded article, additionally a gas is forced in with such a pressure that the material is uniformly distributed over the surface of the mold cavity, so that a hollow body is formed which, while the gas pressure is maintained, is cooled to a temperature which is below the softening point of the plastics material.
In this known method, after forcing-in the gas, the gas inlet opening of the hollow body is closed by after-injecting a sufficient amount of molten material and is kept closed until the plastics material is cooled down below its softening point. After cooling, the gas chamber of the hollow body is opened by drilling or punching a hole in the body in order to achieve a pressure release to the atmosphere and only subsequently is the hollow body removed from the injection mold after the injection mold has been opened.
The above-described method makes it possible to influence the filling procedure of the injection mold in such a way that the hollow body is formed in the injection mold and the thermoplasticity of the plastics material is fixed in its shape by the gas pressure.
However, it has been found that, when hollow bodies of plastics material are injection molded in accordance with this known method, the structure and/or density of the walls of the hollow bodies are not always optimal, therefore, the usefulness of the finished molded articles for their purpose could be more or less diminished.
It is, therefore, the primary object of the invention to provide a method for injection molding articles of thermoplastic material and an apparatus for carrying out this method which makes it possible to influence in accordance with different environments the structure and density of the plastics material forming the walls of the molded articles while the injection procedure is carried out.